Web tension control system

ABSTRACT

A system for use in controlling the substitution of a replacement roll of paper for an expiring roll of paper, while maintaining essentially uniform tension conditions in the paper web forwarded to a web printing machine. The system utilizes dual web tension responsive controls for controlling web tension during the roll replacement or transition stage, and provides an emergency stop control for arresting rotation of both of the replacement and expiring rolls.

1, May 28, 1974 ilnited States Fatent [1 1 Swann et a1.

Roesen Blessing o e .m k .w W 27 56 99 NH 56 96 88 66 92 23 .m r a t mi k M em i. Em w m FW m an T WH N GN C m N mm mrflau MCCB E 1 T m B n E w W .m r ms o 5 r Primary ExaminerDonald E. Watkins Assistant ExaminerJohn M. .lillions [73] Assignee: Arcata Graphics Corporation,

Depew, N.Y. Attorney, Agent, or Firm--Bean & Bean Apr. 13, 1973 [22] Filed:

[57] ABSTRACT A system for use in controlling the substitution of a replacement roll of paper for an expiring roll of paper, while maintaining essentially uniform tension condi- [21] Appl. No.: 351,098

tions in the paper web forwarded to a web printing machine. The system utilizes dual web tension respon- AM 5H H 2 3M & 26

2 ,2, .0 3 7W yll/l h& 96/4 382 8 .ME 4 m 2 m Th wm mmw "W LM om SM UIF sive controls for controlling web tension during the roll replacement or transition stage, and provides an emergency stop control for arresting rotation of both of the replacement and expiring rolls.

[56] References Cited UNITED STATES PATENTS 2,070,327 2/1937 Wood 242/75.4l 3 Claims, 4 Drawing Figures PATENTEDMAY 28 I974 TENS/01V SHEET 2 UF 3 OPEIPETORS-TOGGLE F g MAl/vnm 5x L/NKTO SUPPLY FLOAT/N6 ROLLER 75' 2 4:7 )2 q 1 STOP VALVE 1 QUICK EXH.

CYLS

)j I I o W TRANSIT/0N TENSION 30L. VALVE "'1 EXH.

w BUTT BRAKE SOLENOID QQ U VALVE 2' q f 8 J76 7? ROLL LOCK VALVE EXH.

"PRIOR ART- LORD/N6 POSITION EX FIRING ROLL PATENTEI] MAY 2 8 I974 OPERATORS T0 GGLE sxu. E

N l/N fill? SUPPLY Fig.3.

FLOHT/NG ROLLER LIN/(TO STOP VALVE QUICK EXH- 'gl I 78 REG G 6 TRANSIT/0N g TESNSION OL. .90 2 g 1 1 4 VALVE fk 5g 96 2 BUT T BRRKE SOLENOID f J 40 j 1 ROLL LOCK LOHDI/ G VALVE POSITION I E) (PIRlN6 kOLL EXH TIME WEB TENSION CONTROL SYSTEM BACKGROUND OF THE INVENTION Feeding devices for use in feeding a web of paper to a web printing machine normally comprise a roll stand or reel having a pair of rotatable spiders whose arms carry chucks for releasably engaging opposite ends of an expiring and one or more replacement rolls; a tension control mechanism including a plurality of driven tension belts for maintaining tension in a web withdrawn by the printing machine from the expiring roll; an auxiliary roll drive for bringing the peripheral speed of a replacement roll up to the speed of the web; and a mechanism for pasting or splicing an adhesively coated leading end of the web on the replacement roll to the web passing from the expiring roll and immediately thereafter severing the web of the latter.

In operation of these prior feeding devices, when the expiring roll reaches some given diameter, an operator depresses a cycle start button, which serves to rotate the reel to position the replacement roll in a pasting position closely adjacent the running web; to operate the auxiliary drive in order to accelerate the replacement roll up to web speed; and to position a pasting and severing mechanism adjacent its operating position. When the expiring roll has become substantially expired, the operator depresses a splice button, which initially causes the pasting and severing mechanism to press the web against the replacement roll in order to effect a splice and immediately thereafter to sever the web between the splice and the expiring roll. After the web is severed, the pasting and splicing mechanism and the auxiliary drive are removed from their operating positions and the reel is further rotated to position the replacement roll in the normal web runof position previously occupied by the expiring roll. A suitable pneumatically operated transition brake is normally as sociated with the replacement roll in order to supply transition tension to the web simultaneously with the act of severing the web; this transition brake being maintained ON until the replacement roll has reached the run-of position and the web is under the sole control of the tension belts. When the replacement roll is in run-off position, the expiring roll or butt is removed from the reel and another replacement roll is loaded in its place.

With present commercial systems constructed in accordance with the general teachings of U.S. Pat. No. 2,596,] 89, a single valve controlled by a floating roller is employed to control supply of compressed air to both tension cylinders, which control tension in the tension belts, and to the transition brake associated with the replacement roll. A problem is encountered with present systems, due in part to the fact that it has been found that the air pressure required by the transition brake is often less than that required by the tension cylinders to maintain the same web tension. Thus, when air is supplied to the transition brake and the transition brake goes ON," there tends to be a rapid rise in web tension to which the floating roller responds for the purpose of lowering the applied air pressure to a value required by the transition brake to maintain a desired web tension. As a result, the web is subjected to a rapidly changing tension condition termed a tension spike, which may under certain conditions produce a tearing or severing of the web. Further, it

will be understood that, since the same reduced air pressure is also supplied to the tension cylinders, the belt tension is correspondingly reduced to a value substantially below that required to maintain web tension when the belts are the sole controlling factor.

Also, it will be understood that, as the replacement roll is moved towards the run-of position, the web is stretched and is thus subject to additional tension, which is sensed by the floating roller. As a result there tends to be an additional decrease in air pressure supplied to the tension cylinders, as well as the transition brake. Thus, when the transition brake is turned OFF, as the replacement roll approaches the runof position, the belt tension is substantially below that required to maintain a normal web tension condition and the web tends to run away. The immediate resultant slack or run-away condition of the web is sensed by the floating roller and full air pressure is immediately supplied to the tension cylinders; this resulting in the production of a large and extremely rapid increase in web tension. As time progresses, the web tension is of course reduced to its normal value, but ofttimes the tension spike resulting from the sharp increase in belt tension is sufficient to break the web. It will be understoodthat this tension spike is normally substantially in excess of the tension spike produced when the web is severed and the transition brake of the replacement roll goes ON.

SUMMARY OF THE INVENTION In accordance with the present invention the drawbacks of the above described commercial system are overcome by connecting the transition brake into the air supply independently of the floating" roller controlled valve and placing a cam controlled dump valve immediately upstream of the transition brake. The dump valve is mechanically coupled to the floating roller such that when the floating roller senses an increase in web tension air is dumped from the transition brake circuit. A manually adjustable bleed valve is arranged between the dump valve and the transition brake in order to bleed a small percentage of air from the latter whereby to reduce shock occasioned by introduction of air to the brake, and thus cushion shocks I to which the web is exposed incident to the braking operation. The use of this bleed valve further reduces hunting of this system.

With the present arrangement, that portion of the second tension spike occasioned by turning ON the transition brake is substantially reduced, since the air pressure initially applied to the transition brake may be adjusted independently of the floating roller controlled valve in order to provide for proper web tension conditions. The small spiking effect or tension curve discontinuity noted at this point, is believed to be primarily occasioned by the web severing operation.

As the reel is rotated to position the replacement roll in run-of position, the resultant slight increase in web tension occasioned by stretching of the web is sensed by the floating roller and the dump valve operated to reduce pressure on the transition brake in order to maintain essentially constant web tension conditions.

Since the transition brake and the floating" roller controlled valve supplying the tension cylinders are furnishedwith separate air supplies, the belt tension tends to remain more or less constant during the transition stage, and thus when the transition brake is turned OFF, the tension belts are immediately effective to resume substantially normal tension control of the web. A slight tension curve discontinuity noted as normally occurring at this point in the operational cycle is far below that which will cause the web to tear or rip.

DRAWINGS The nature and mode of operation of the present invention will now be more fully described in the following detailed description taken with the accompanying drawings wherein:

FIG. 1 is a view of a web feeding device with which the control system of the present invention possesses utility;

FIG. 2 is a diagrammatic view illustrating a control system previously employed with web feeding devices of the type illustrated in FIG. 1;

FIG. 3 is a diagrammatic view illustrating the control system of the present invention; and

FIG. 4 is a graphic comparison of web tension conditions encountered when utilizing the control systems of FIGS. 2 and 3.

DETAILED DESCRIPTION Reference is now made particularly to FIG. 1, wherein a web feeding device is generally designated as and shown as including a reel or roll stand 12 arranged in association with a web tension control mechanism 14, a web pasting and severing mechanism 16, and a replacement roll or auxiliary drive mechanism 17.

Reel 12 is formed with a pair of spiders 18, which are supported on and spaced apart axially of a common shaft or axis 20. Although only one of the spiders is illustrated, it will be appreciated that each includes a plurality of arms 22, which carry chucks, not shown, for rotatably supporting opposite ends of a plurality of paper rolls. For purposes of reference, these rolls are designated as being an expiring roll 24, a replacement or new roll 26, and a second replacement roll 28; and reel 12 is shown in full line as being in its rotatable position at which expiring roll 24 is disposed in a normal web run-off position. When expiring roll 24 is disposed in run-of position, a web of paper 30 is withdrawn therefrom by web forwarding mechanism associated with a printing machine, not shown; web 30 while in transit to the printing machine passing over a plurality of guide rolls 32 and a floating web tension sensing roller 34 and being maintained under desired tension by mechanism 14.

Web tension control mechanism 14 includes a plurality of driven, transversely spaced endless belts 36, which are arranged in face to face frictional engagement with the surface of web 30, as it is withdrawn from expiring roll 24. The tension of belts 36 and thus the resultant tension in web 30, is controlled by one or more tension cylinders 38, which are in turn under the control of a flow control valve 40 mechanically associated with floating roller 34. Roller 34 is supported for vertically directed swinging movements by a mounting bar 42 journaled' on a pin shaft 44; a free end of bar 42 carrying a force transmitting roller 46 engageable with a movable part 40a of valve 40.

Valve 40, which is of conventional construction, generally includes a spring biased diaphragm carrying an exhaust valve seat, and a double headed exhaust valve stem spring biased to normally close a stationary air inlet valve seat arranged in alignment with the movable exhaust valve seat. The arrangement is such that when the floating roller moves to the left, as viewed in FIG. 1, as a result of a decrease in web tension, roller 46 exerts pressure on part 40a and thus on the top of the pressure regulating spring and diaphragm. As the diaphragm moves downwardly, the exhaust valve seat pushes the exhaust valve stem off its inlet valve seat and air from an inlet conduit 48 flows through valve 40 and out to cylinders 38 through conduit 50, whereby the cylinders are extended to effect an increase in the tension in belts 36.

When. the air pressure being delivered to tension cylinders 38 overcomes the mechanical force being exerted on the top of the diaphragm, the diaphragm lifts and the valve stem closes against the inlet valve seat, while the exhaust valve seat remains in a closed condition, this cutting off further supply of air pressure to the tension cylinders while preventing any escape of air pressure through the exhaust conduit 52. This is the condition of valve 40 when floating" roller 34 senses a desired web tension condition.

When floating roller 34 moves to the right in response to an undesired increase in web tension, there results a reduction of the mechanical force on the top of the diaphragm, and air pressure below the diaphragm overcomes the mechanical force on top of it and the diaphragm raises. When this occurs, the inlet valve remains closed, but the exhaust valve seat is moved sufficiently far to permit same to open in order to exhaust air pressure from the tension cylinders through conduit 52 until such time as the air pressure below the diaphram again balances the mechanical force on top of it.

Mechanism 16 is of conventional construction and generallyincludes separate -brush" and knife devices, not shown. The brush device is operable to force web 30 intoweb splicing engagement with an adhesive surface provided on the leading end of a web carried by replacement roll 26, when reel 12 is rotated to move the replacement roll'into the position illustrated in phantom line in FIG. 1; and the knife device is subsequently operable to extend between belts 36 for the purpose of severing web 30 intermediate the splice and expiring roll 24.

Mechanism 17, which is of conventional construction, generally includes a power driven belt or belts selectively movable into engagement with the surface of replacement roll 26 for the purpose of bringing same up to the running speed of web 30 prior to operation of mechanism 16 to effect splicing of web 30.

Reference is now made particularly to FIG. 2, which illustrates a portion of a conventional control system used in conjunction with a prior web feeding device of the type thus far described. In addition to the previously described tension cylinders 38 and valve 40, this system includes an air commutator 60 arranged in association with reel 12 to control feeding of air to brake devices, generally designated at 24a, 26a and 280, which are employed for selectively braking or retarding I rotation of rolls 24, 26 and 28, respectively when reel 12 is in different rotatable positions thereof. Commutator 60 is selectively supplied with three separate air supplies, namely, a replacement roll transition brake or first air supply established by a conduit 62 connected in flow communication with tension cylinder control.

conduit 50, which is under the control of a solenoid operated transition control valve 64; an expiring roll brake or second air supply established by a conduit 66 connected in flow communication with a common system air source 68, which is under the control of a pressure regulator 70 and a butt brake solenoid operated valve 72; and a second replacement roll or third air supply established by a conduit 74 connected in communication with conduit 66 intermediate regulator 70 and valve 72, which is under the control of a manually operated lock valve 76.

Operation of the above described prior system will now be discussed with reference to FIGS. 1, 2 and 4; it being assumed that reel 12 is initially in the full line position illustrated in FIG. l with web 30 being withdrawn from expiring roll 24. At this point-in time, the tension in web 30 is controlled by operation of valve 40 in response to web induced movements of floating roller 34. When expiring roll 24 has reached some given diameter, an operator will depress a cycle start button, not shown, which serves to rotate reel 12 into the phantom line position illustrated in FIG. 1, wherein replacement roll 26 is disposed in a pasting position closely adjacent web 30. Simultaneously therewith, mechanism 17 is operable to accelerate replacement roll 26 up to a speed essentially matching the running speed of web 30 and mechanism 16 is moved into an operating positionclosely adjacent the running web. When expiring roll 26 has become substantially expired or depleted, the operator presses a splice button, not shown, which serves to initially cause the brush device of mechanism 16 to press web 30 against the surface of replacement roll 26 in order to effect a splice; such splicing operation effecting a slight stretching of and/or braking action on web 30, resulting in the production of a small tension spike, which is designated as A in the web tension-time curve illustrated in FIG. 4. Immediately thereafter, the knife device of mechanism 16 is fired to sever web 30 between the splice and expiring roll 24. Simultaneously therewith, valve 64 is opened to place conduit 62 in flow communication with conduit 50 with the result that the transition brake 26a is turned ON. The web severing operation and the turning ON of transition brake 26a result in the production of an enlarged spike designated as B in the web tension-time curve, which under certain conditions is sufficiently large to effect breaking or severing of the web 30 now being withdrawn from replacement roll 26.

After the web between the splice and expiring roll 24 is severed, mechanism 16 and 17 become inoperative and reel 12 is further rotated to position replacement roll 26 in the run-of position initially occupied by expiring roll 24. As replacement roll 26 approaches the run-off position, transition brake 26a is turned OFF, i.e., valve 64 operated to vent the transition brake to the atmosphere, and the tension in web 30 is thereafter under the sole control of tension belts 36. At this point in time there is experienced an excessively large tension spike designated as C" in FIG. 4, which quite often effects breaking or severing of the web even under otherwise ideal operating conditions.

After the web severing operation described above, valve 72 is automatically operated in order to arrest rotation of the now expired or butt roll 24, during the period of time reel 12 is rotated to move roll 26 from its pasting into the run-of position. After reel rotation is completed, the now expired roll occupies the position previously occupied by roll 28; roll 28 having been moved into the original position of roll 26 to await the next splicing and severing operation. The expired roll may now be removed and replaced by another replacement roll; this operation being facilitated by manually operating valve 76 in order to momentarily lock the expired roll and/or new replacement roll against rotation.

After extensive investigation, it has been found that the pronounced tension spikes B" and C are caused in part by the fact that the value of the air pressure required by brake 26a, associated with replacement roll 26, is substantially less than thatrequired by tension cylinders 38 to maintain the same web tension. Thus, when air is supplied to brake 26a and the brake goes ON, there tends to be a rapid rise in web tension, i.e., tension spike B to which floating roller 34 responds for the purpose of lowering the pressure of air in conduit 50 to a value required by brake 26a to maintain desired web tension. The substantial reduction in air pressure within conduit 50 causes a corresponding reduction in the tension applied to belts 36 by cylinders 38. Accordingly, when replacement roll 26 is moved into run-off position and transition brake 26a goes OFF," belts 36 are initially ineffective for controlling web tension and the web tends to become slack and run-away. Of course, this reduction in web tension is immediately sensed by floating roller 34 and high pressure applied through conduit 50 to cylinders 38. However, the resultant large and rapid increase in belt tension, produces a correspondingly large and rapid increase in web tension, which is in the form of tension spike C.

In accordance with the present invention the previously described system is modified in the manner illustrated in FIG. 3, by connecting conduit 62 into common supply conduit 68 and incorporating a pressure regulator 78 in conduit 62 upstream of valve 64, in order to reduce available air pressure to a value providing for proper operation of transition brake 26a. Further, a branch conduit 80, which incorporates a dump valve 82 is connected into conduit 62 downstream of valve 64; and a branch conduit 84, which incorporates a manually adjustable air bleed valve 86, is connected into conduit 62 intermediate conduit 80 and commutator 60.

Again referring to FIG. 1, it will be understood that the condition of dump valve 82 is controlled by a cam operator 88, which is fixed for rotation with mounting bar 42 about the axis of pin shaft 44.

The overall mode of operation of the system thus far described in connection with FIG. 3 is similar to that discussed above in connection with FIG. 2, except that when transition brake 26a goes ON the air pressure initially applied thereto is of a value required by such brake to maintain a proper web tension condition; bleed valve 86 serving to cushion the shock of operation of the brake. As a result, the tension spike associated with this part of the operational cycle is due essentially to the operation of the knifedevice and is of an acceptable value, as indicated at B in FIG. 4.

During further rotation of reel 12 to position roll 26 in the run-off position, the tendency for the tension in web 30 to increase, due to slight stretching of the web, is sensed by the floating roller, with the result that dump valve 82 is progressively opened by cam operator 88 as required to reduce the pressure of air supplied to brake 26a and thus maintain web tension essentially constant. It will of course be understood that slight movements of the floating" roller necessary to control operation of dump valve 82 produce only slight adjustments of valve 40 and thus the pressure of air supplied to cylinders 38 is also maintained essentially constant and at essentially its proper value even though the pressure of air supplied to brake 26a is reduced. As

a result, when brake 26a is turned OFF," belts 36 are substantially immediately effective to properly tension the web. While system factors tend to produce a slight web tension curve discontinuity at this point in time, which is designated as C in FIG. 4, such discontinuity is well within acceptable limits.

The web tension curve illustrated in FIG. 4 is not meant to represent any actual curve obtained from tests of the prior and present systems. Rather, it is meant to illustrate the number and relative magnitude of tension spikes" occuring during the roll change operation with each system, and to indicate that a sub-v stantial difference in magnitude has been noted over the testing period between the values of tension spikes Again referring to FIG. 3, it will be appreciated that the present system additionally features a novel system modification permitting emergency braking of roll 24 during the-normal web feeding operation; and of rolls 24 and 26, during the roll change operation in the event the web breaks or its motion is otherwise arrested within the printing apparatus. This modification includes the provision of a conduit 90 including a pressure regulator 92 and an emergency responsive, solenoid operated stop valve 94, wherein conduit 90 is connected into conduit 62 via a branch conduit 96 incorporating a check valve 98 and directly into conduit 66. Further, a check valve 100 is provided in conduit 90 intermediate branch conduit 96 and conduit 66; and a branch conduit 102 is connected into conduit 66 down stream of butt brake solenoid valve 72 and incorporates an air bleed valve 104. While valve 94 could of course be manually operated, it is preferably under the automatic control of web condition sensing devices incorporated within the printing apparatus.

If a web break is detected, valve 94 is opened and high pressure air is immediately supplied to conduits 62 and 66. If rolls 24 and 26 are within their normal range of movements incident to a roll change operation, brakes 24a and 26a are immediately turned ON to arrest roll rotation. If roll 24 is in run-of position, only brake 24a is turned ON. Check valves 98 and 100 serve to prevent coupling or flow communication between conduits 62 and 66, and air bleed valve 104 permits bleed off of air from conduit 66 downstream of valve 72 when valve 94 is rendered inoperable.

We claim:

l. In a system for controlling operation of a device for feeding a web to a printing machine, wherein said device includes a rotatable reel for supporting an expiring roll from which said web is withdrawn for passage to said printing machine when said expiring roll is in run-of position and at least one replacement roll movable upon rotation of said reel successively through nism for splicing said web passing from said expiring roll to a leading end of a web carried by said replacement roll upon movement of the latter into said past-, ing position and thereafter severing said web between a resultant splice and said expiring roll; a web tensioning mechanism arranged for surface engagement with said web when withdrawn from said expiring roll; a transition brake associated with said replacement roll for applying transition tension to said web withdrawn therefrom after severing of said web as aforesaid and until said replacement roll is disposed in said run-off position; web tension sensing means movable in response to changes in web tension; and a pneumatic control circuit means responsive to movement of said web tension sensing means for adjustably controlling the pressure of air supplied to said web tensioning mechanism and said transition brake whereby to vary web tension, characterized in that the pressure of air required by said transition brake to maintain a given web tension is substantially less than that required by said web tensioning mechanism to maintain said given web tension, the improvement wherein said controlcircuit means comprises:

first conduit means for connecting said web tensioning mechanism in flow communication with a source of air under a given pressure and incorporating a valve responsive to movements of said web tension sensing means for adjustably controlling pressure of air applied to said web tensioning mechanism;

second conduit means for separately connecting said transition brake to said source and incorporating means for reducing pressure of air available for supply to said transition brake to a given value sub stantially below said given pressure; and

a branch conduit connected into said second conduit means intermediate said pressure reducing means and said transition brake, said branch conduit incorporating a dump valve controlling communication of said branch conduit with the atmosphere for reducing pressure applied to said transition brake to a value below said given value, said dump valve being responsive to said web tension sensing means.

2. In a system according to claim 1, wherein an other branch conduit is connected into said second conduit means intermediate said branch conduit and said transition brake, said other branch conduit incorporating a valve for slowly bleeding air from said second conduit means to the atmosphere.

3. A system for controlling operation of a device for feeding a web to a printing machine, said device comprising in combination:

a rotatable reel for supporting an expiring roll from which said web is withdrawn for passage to said printing machine when said expiring roll is in runof position and at least one replacement roll movable upon rotation of said reel successively through a pasting position and into said runof position to replace said expiring roll when said web is substantially expired therefrom;

web tension sensing means movable in response to changes in web tension;

a web splicing and severing mechanism for splicing said web passing from said expiring roll to a leading and thereafter severing said web between a resultant splice and said expiring roll;

a web tensioning mechanism arranged for surface engagement with said web when withdrawn from said expiring roll;

first circuit means for connecting said web tensioning mechanism to a source of air under a given pressure, said first circuit means including flow control valve means responsive to movements of said web tension sensing means for adjustably varying operation of said web tensioning means;

a transition brake associated with said replacement roll for applying transition tension to said web withdrawn therefrom incident to the severing of said web as aforesaid and during transitional movement of said replacement roll into said run-off position;

a second circuit means for connecting said transition brake directly to said source incident to severing of said web and during transitional movement of said replacement roll as aforesaid, said second circuit means including means for reducing pressure of air available to said transition brake to a value below said given pressure and second valve means connected into a portion of said second circuit means intermediate said pressure reducing means and said 1 transition brake, said second valve means being responsive to web tension increase sensing movements of said web tension sensing means for connecting said portion of said second circuit means to the atmosphere whereby to further reduce pressure of air supplied to said transition brake;

an expiring roll brake;

a third circuit means including third valve means operable for connecting said expiring roll brake to said source after the severing of said web and removal of said expiring roll from said run-of position; and

fourth circuit means connected into said source and operable in response to a breaking of said web to supply braking air to said portion of said second circuit means and to said third circuit means intermediate said third valve means and said expiring roll brake, whereby rotation of said expiring roll is arrested when said web breaks while said expiring roll is in said run-of position and rotation of said replacement roll is arrested when said web breaks during said transitional movement, said fourth circuit means incorporating check valve means for normally blocking communication of said second and third circuit means with said fourth circuit means and for blocking communication between said second and third circuit means. 

1. In a system for controlling operation of a device for feeding a web to a printing machine, wherein said device includes a rotatable reel for supporting an expiring roll from which said web is withdrawn for passage to said printing machine when said expiring roll is in ''''run-off'''' position and at least one replacement roll movable upon rotation of said reel successively through a ''''pasting'''' position and into said ''''run-off'''' position to replace said expiring roll when said web is substantially expired therefrom; a web splicing and severing mechanism for splicing said web passing from said expiring roll to a leading end of a web carried by said replacement roll upon movement of the latter into said ''''pasting'''' position and thereafter severing said web between a resultant splice and said expiring roll; a web tensioning mechanism arranged for surface engagement with said web when withdrawn from said expiring roll; a transition brake associated with said replacement roll for applying transition tension to said web withdrawn therefrom after severing of said web as aforesaid and until said replacement roll is disposed in said ''''run-off'''' position; web tension sensing means movable in response to changes in web tension; and a pneumatic control circuit means responsive to movement of said web tension sensing means for adjustably controlling the pressure of air supplied to said web tensioning mechanism and said transition brake whereby to vary web tension, characterized in that the pressure of air required by said transition brake to maintain a given web tension is substantially less than that required by said web tensioning mechanism to maintain said given web tension, the improvement wherein said control circuit means comprises: first conduit means for connecting said web tensioning mechanism in flow communication with a source of air under a given pressure and incorporating a valve responsive to movements of said web tension sensing means for adjustably controlling pressure of air applied to said web tensioning mechanism; second conduit means for separately connecting said transition brake to said source and incorporating means for reducing pressure of air available for supply to said transition brake to a given value substantially below said given pressure; and a branch conduit connected into said second conduit means intermediate said pressure reducing means and said transition brake, said branch conduit incorporating a dump valve controlling communication of said branch conduit with the atmospheRe for reducing pressure applied to said transition brake to a value below said given value, said dump valve being responsive to said web tension sensing means.
 2. In a system according to claim 1, wherein an other branch conduit is connected into said second conduit means intermediate said branch conduit and said transition brake, said other branch conduit incorporating a valve for slowly bleeding air from said second conduit means to the atmosphere.
 3. A system for controlling operation of a device for feeding a web to a printing machine, said device comprising in combination: a rotatable reel for supporting an expiring roll from which said web is withdrawn for passage to said printing machine when said expiring roll is in ''''run-off'''' position and at least one replacement roll movable upon rotation of said reel successively through a ''''pasting'''' position and into said ''''run-off'''' position to replace said expiring roll when said web is substantially expired therefrom; web tension sensing means movable in response to changes in web tension; a web splicing and severing mechanism for splicing said web passing from said expiring roll to a leading end of a web carried by said replacement roll upon movement of the latter into said ''''pasting'''' position and thereafter severing said web between a resultant splice and said expiring roll; a web tensioning mechanism arranged for surface engagement with said web when withdrawn from said expiring roll; first circuit means for connecting said web tensioning mechanism to a source of air under a given pressure, said first circuit means including flow control valve means responsive to movements of said web tension sensing means for adjustably varying operation of said web tensioning means; a transition brake associated with said replacement roll for applying transition tension to said web withdrawn therefrom incident to the severing of said web as aforesaid and during transitional movement of said replacement roll into said ''''run-off'''' position; a second circuit means for connecting said transition brake directly to said source incident to severing of said web and during transitional movement of said replacement roll as aforesaid, said second circuit means including means for reducing pressure of air available to said transition brake to a value below said given pressure and second valve means connected into a portion of said second circuit means intermediate said pressure reducing means and said transition brake, said second valve means being responsive to web tension increase sensing movements of said web tension sensing means for connecting said portion of said second circuit means to the atmosphere whereby to further reduce pressure of air supplied to said transition brake; an expiring roll brake; a third circuit means including third valve means operable for connecting said expiring roll brake to said source after the severing of said web and removal of said expiring roll from said ''''run-off'''' position; and fourth circuit means connected into said source and operable in response to a breaking of said web to supply braking air to said portion of said second circuit means and to said third circuit means intermediate said third valve means and said expiring roll brake, whereby rotation of said expiring roll is arrested when said web breaks while said expiring roll is in said ''''run-off'''' position and rotation of said replacement roll is arrested when said web breaks during said transitional movement, said fourth circuit means incorporating check valve means for normally blocking communication of said second and third circuit means with said fourth circuit means and for blocking communication between said second and third circuit means. 